In making tires and other rubber products, it is desirable to mix silica with an elastomer or rubber to improve certain properties of the elastomer. It is well known to incorporate silica into rubber using a dry mixing process, where a material is put on the surface of the silica during the mixing process to allow it to blend into the rubber. When the silica is coated with such an agent, the silica is referred to as hydrophobated, and any material used to make hydrophobated silica is a hydrophobating agent.
A variety of silane compounds have been developed as hydrophobation agents. Known silane compounds and processes for incorporating silica into rubber are described in U.S. Pat. No. 8,357,733 to Wallen et al.
One known class of silane is mercapto silane, which has an active thiol group and offers excellent coupling between rubber and silica. A commercially available mercapto silane, which has desirable water solubility when hydrolyzed, is 3-mercaptopropyl trimethoxy silane, having the following structure.

A disadvantage of using mercapto silane as a hydrophobating agent is that it tends to contribute to poor scorch resistance or scorch time, in both conventional rubber compounds and when used in the silica masterbatch process. Scorch is a reflection of the fully compounded rubber's ability to be thermally processed without premature vulcanization or crosslinking, and it is a very important parameter in processing rubber. As the rubber begins to crosslink, it can no longer be extruded and/or formed into a useful article. Thus, longer scorch times are desirable. Rubber compounds with longer scorch times can be processed at higher temperature, and can be reworked more than rubber with shorter scorch times. Compounds with longer scorch times can significantly improve tire plant productivity.
Blocked mercapto silanes are also known, in which the thiol group undergoes a preliminary reaction with another chemical constituent to become essentially unreactive under normal mixing conditions, but when heated to a higher temperature will react as though the thiol group were present in its original condition. The processing behavior of blocked mercapto silanes is very good. However, the use of known types of blocked mercapto silane is cost prohibitive in many rubber applications.
There is a continuing need for a method by which mercapto silane may be used as a hydrophobating agent in rubber compounds and a silica masterbatch process. Desirably, the method permits the use of commercially available mercapto silane while providing sufficient scorch resistance.